I have a 2000 JGC laredo that has the 4.0 straight 6cyl. A few months ago, it overheated because of the fan relay switch. I replaced the switch, and the fans started working again. It still gets very hot in the summer, around 270degrees, I have to turn on the heater to get it cool down, but as soon asI turn on the A/C, it heats up again. I have replaced the head gasket, even though it wasnt cracked, thermostat, water pump, flushed the radiator, but it still over heats. Anyways, back to the fuel mileage, ever since the overheating, I have have been getting around 12mpg, before, I was getting 19mpg. This thing has me so pissed off,I cant even see straight. I have performed a fuel inj cleaning, new plugs, air filter, cleaned the throttle body. Pretty much every thing I can think of. can anyone please help, I am even a mechanic, and I cant figure this out. I was thinking catalytic converter, but doesnt really shows signs of being bad. please help.

Mileage is about right for city driving.
270 degrees?! That should peg the gauge. I would say the electric fan -- you have to make sure it transitions to high rpm when additional cooling is needed.

I have a 2000 JGC laredo that has the 4.0 straight 6cyl. A few months ago, it overheated because of the fan relay switch. I replaced the switch, and the fans started working again. It still gets very hot in the summer, around 270degrees, I have to turn on the heater to get it cool down, but as soon asI turn on the A/C, it heats up again. I have replaced the head gasket, even though it wasnt cracked, thermostat, water pump, flushed the radiator, but it still over heats. Anyways, back to the fuel mileage, ever since the overheating, I have have been getting around 12mpg, before, I was getting 19mpg. This thing has me so pissed off,I cant even see straight. I have performed a fuel inj cleaning, new plugs, air filter, cleaned the throttle body. Pretty much every thing I can think of. can anyone please help, I am even a mechanic, and I cant figure this out. I was thinking catalytic converter, but doesnt really shows signs of being bad. please help.

You might try a compression check. With the temps getting that high, you may have caused the rings to lose their tensil strength, which would give you a loss of compression, not firing fully and throwing some of the fuel right out the exhaust cylinder. You didn't mention plug wires either. But lower compression could cause your engine to work a lot harder, maybe even causing it to run hot. Good luck.

I tried a compression test, I got 135psi #1 cyl, 140psi #2, and 165psi the rest of them. The was the reason I checked the head gasket, Because I thought I might have a crack between 1 and 2. no crack found, and I am not losing coolant. Also, I made a mistake on the temp guage, I usually reads 145degrees, then I turn on the heater. It would probally get hotter if I didnt turn the heater on. I was thinking the radiator might be stopped up, but I flushed it and reversed flushed, didnt seem like it was stopped up. I was just wondering about the fan relay switch, I wonder if it has 2 speeds, and the ones you get from NAPA just have 1 speed. I just looked today, now I have 2 leaking rear axle seals, and a leaking rear main seal, it just keeps getting better and better.

The fan speed is controlled by the fan relay module and PCM, not the fan itself.
usually reads 145degrees
That's way too cold - should read 210* give or take a few.

My fault, I meant 245 degrees. Sorry, work is getting to me.

Hi I am having the same problem with the milage and heating if you get to the bottom of this will you please let me know the cure = 2 new head gaskets = new head = old head tested out ok and replaced with another head gasket = along with all the other things you have done plus new high flow air intake = I have seen a High output water pump add on ebay so ??? do I or dont I its cost a fortune so far = your Boab